1. Field of the Invention
The present invention relates to a projection optical system, in particular an micro-lithographic projection optical system.
2. Brief Description of Related Art
Lithographic processes are commonly used in the manufacture of semiconductor elements, such as integrated circuits (ICs), LSIs, liquid crystal elements, micropatterned members and micromechanical components.
A projection exposure apparatus used for photolithography generally comprises an illumination optical system having a light source and a projection optical system. Light from the illumination optical system illuminates a reticle (a first object) having a given pattern, and the projection optical system transfers an image of the reticle pattern, onto a region of a photo-sensitive substrate (a second object). The image of the reticle pattern may also be reduced in size by the projection optical system so as to produce a smaller image of the reticle pattern on the substrate.
The trend to ever smaller and more sophisticated miniaturized devices places increasingly high demands on the projection exposure systems and thus projection optical systems used for the manufacture of these devices. In order to achieve higher resolutions in the exposure of substrates, the imaging of the reticle onto the substrate has to be performed with a sufficiently high numerical aperture (NA) on the side of the substrate. Therefore, an increase of the numerical aperture is a decisive factor in the development of improved projection exposure systems.
Projection exposure systems having a high numerical aperture are known from US 2003/0007253 A1 and WO 2003/075049 A2 and WO 2005/054956 A2 which documents are incorporated herein by reference.
Some conventional projection exposure systems are able to achieve a numerical aperture of greater than 1. One example of such projection exposure systems is referred to as a projection exposure system of the immersion type, known for example from WO 2003/077037 A1, which document is incorporated herein by reference. Another example of such projection exposure system is referred to as a projection exposure system of the near field exposure type or solid immersion type, known for example from WO 2003/077036 A1, which document is incorporated herein by reference.
High numerical apertures also bring about a whole range of challenges in terms of a design of the projection optical system. In purely refractive optical systems for projection exposure the requirements for correction of imaging errors, such as aberrations and the like, are increasing with increasing numerical aperture on an image side of the optical system. In addition to the demand for a well corrected wavefront, parameters such as a telecentricity on a reticle side need to be taken into account. These requirements are typically met by using aspherical lenses close to an image side of the projection optical system.
Projection optical systems tend to increase in weight and size as the numerical aperture of such systems increases.
In particular, diameters of lenses increase to such an extent that they become very expensive and difficult to manufacture, and the manufacture of aspherical lenses of a high diameter and a sufficient accuracy presents particular problems.